(1) Field of the Invention
The present invention relates to an optical amplifier which is suitably used for a wavelength multiplexed optical transmission system.
(2) Description of the Related Art
In a trunk line optical communication system, with the attainment of a high speed designed for lengthening a distance and increasing a capacity in recent years, optical modulators and electronic circuits which can deal with such a situation have been developed. However, it has been extremely difficult to provide an electronic circuit to be used in a region of 10 Gb/s or higher. Accordingly, a study has also been made on a system which can attain large capacitance for the transmission of an optical signal by using a wavelength multiplexing technology.
Referring to FIG. 26, there is shown a typical wavelength multiplexed optical transmission system. This wavelength multiplexed optical transmission system denoted by a code 80 includes terminal stations 20A, 20B and 20c, a repeater station 20D and optical amplifiers 20a to 20f.
The terminal stations 20A, 20B and 20C are points in which transmitting and receiving of information are performed through an optical fiber.
Each of the terminal stations has transmitting and receiving units. The repeater station 20D performs a relaying operation according to information regarding a terminal station to which information should be transmitted from a certain terminal station. For example, the repeater station 20D has a signal branching unit or a signal dividing unit, and divides an optical signal having certain wavelength information from the terminal station 20A into portions having wavelength information appropriate for the terminal stations 20B and 20C.
The optical amplifiers 20a to 20f amplify optical signals among the terminal stations 20A to 20C interconnected by the optical fiber. The power of a light attenuated during transmission of an optical signal is amplified. The repeater station 20D also has built-in optical amplifiers similar to the optical amplifiers 20a to 20f.
With the wavelength multiplexed optical transmission system 80 shown in FIG. 26, when a multiple optical signal having a plurality of wavelengths is to be transmitted from the terminal station 20A to the terminal stations 20B and 20C, an optical signal having wavelengths of, for instance .lambda.1 to .lambda.4, from the terminal station 20A, is divided into some portions by the repeater station 20D. Then, the portions of the optical signal having wavelengths of, for instance .lambda.1 and .lambda.3, are transmitted to the terminal station 20B, and the portions of the optical signal having wavelengths of, for instance .lambda.2 and .lambda.4, are transmitted to the terminal station 20C. During this period, the portions of the optical signal are amplified by the optical amplifiers 20a to 20f in order to prevent the portions of the signal to be transmitted from being attenuated.
Referring to FIG. 27 which is a block diagram, there is shown an example of a 4-wave multiplex transmitting unit in a typical optical transmission system. This 4-wave multiplex transmitting unit includes light sources 81-1 to 81-4, modulators 82-1 to 82-4, driving circuits 831 to 83-4 and a coupler 85.
The light sources 81-1 to 81-4 output optical signals having specified wavelengths (.lambda.1 to .lambda.4). The modulators (MOD1 to MOD4) 82-1 to 82-4 modulate the optical signals outputted from the light sources 81-1 to 81-4 by signals from the later-described driving circuits 83-1 to 83-4. The coupler 85 synthesizes outputs (multiplexes wavelengths) from the modulators 82-1 to 82-4.
The driving circuits (DRIV1 to DRIV4) 83-1 to 83-4 drive the modulators 82-1 to 82-4 respectively based on main signals (data signals; DATA1 to DATA4).
With the 4-wave multiplex transmitting unit, optical signals having various wavelengths (.lambda.1 to .lambda.4) are modulated by the modulators 82-1 to 82-4.
These modulated optical signals are multiplexed by the coupler 85 and then outputted to the optical amplifiers.
Referring now to FIG. 28 which is a block diagram, there is shown a constitution of a typical optical amplifier. This optical amplifier denoted by a code 90 includes an optical amplifying unit 91, a light branching circuit 92, a light receiver 93, a comparator 94 and a pumping light source control circuit 95. The optical amplifying unit 91 amplifies an optical signal which has been inputted. The inputted optical signal was multiplexed by the coupler 85 in the previous stage. For this optical amplifying unit 91, for instance, a unit composed by combining an erbium doped optical fiber (referred to as EDF, hereinafter) with a pumping light source (referred to as LD; LASER DIODE, hereinafter) for supplying an exciting light to this EDF is used. The light branching circuit 92 branches a portion of the optical signal amplified by the optical amplifying unit 91. This circuit 92 includes, for instance an optical coupler.
The light receiver 93 converts the optical signal branched by the light branching circuit 92 into an electric signal by using a receiving element. The comparator 94 compares an output from the light receiver 93 with a specified reference value (REFERENCE). The pumping control circuit (PUMP LD CONTROL CIRCUIT) 95 receives an output from the comparator 94, adjusts an output from the pumping light source of the optical amplifying unit 91 and corrects its deviation from the reference value.
With the optical amplifier 90 constructed in the above-noted manner, after a portion of an inputted optical signal is branched and compared with a specified value, the gain of the optical amplifying unit 91 is controlled based on the result of this comparison. Accordingly, an average value among lights outputted from the optical amplifier 90 can be maintained constant.
However, there is a problem inherent in the system, which employs optical amplification relaying like that described above. More particularly, since a stable transmission system is realized by always maintaining constant an average value among lights outputted from the optical amplifier 90 and regulating the fluctuation of light receiving power for the terminal stations 20A to 20C, even in the case of N-wavelength multiplex transmission in which a plurality (N) of wavelengths are multiplexed, power for respective wavelengths based on average value control can be maintained constant if the input levels of the wavelengths are the same. However, for example, if a wavelength path is switched to another in the middle way of a transmission line or if the number of wavelengths for an input signal is reduced in the optical amplifier 90 because of a failure or maintenance work, average value control like that described above only results in the increase of output power for the respective wavelengths.
In other words, for N-wavelength multiplex transmission, if the average output power of the optical amplifier 90 is Po, light power per wave for the output of this optical amplifier 90 is PoN. In this condition, if no m waves contained in N waves (m&lt;N) are inputted any longer for one reason or another, light power per wave for the output of the optical amplifier 90 becomes Po/(N-m) and thus power per wave is increased.
To further describe the foregoing problem by taking a 2-wave multiplexing system as an example, assuming that power for each wavelength outputted from the optical amplifier 90 is +6 dBm, when an optical signal inputted to the optical amplifier 90 is reduced from two waves to one wave because of a failure or the like, output power of one wave outputted from the optical amplifier 90 is increased by 3 dB to be +9 dBm. If this power exceeds a threshold value in which an optical fiber nonlinear effect (SBS; stimulated Brillouin scattering, SPM; self phase modulating effect, or the like) is produced, a light waveform is deteriorated and thus transmission quality is also deteriorated.
Efforts have been made to develop a technology as means for solving the above-discussed problem. For example, referring to JP-A-95097/1996, there is disclosed a technology for always keeping a signal light output for each wavelength at a proper level by controlling the light output level of an optical amplifier so as to change the level according to the number of multiple signals in a wavelength multiplexed light signal when the wavelength multiplexed light signal produced by multiplexing an optical signal having a plurality of different wavelengths is to be amplified.
However, with the technology disclosed in JP-A-95097/1996, since the number of wavelength multiplexed light signals is directly detected and the light output level of the optical amplifier is controlled according to this detected number of wavelength multiplexed light signals, means for detecting the number of wavelength multiplexed light signals inevitably becomes complex. Consequently, the optical amplifier as a whole becomes complex and costs are increased.